The present invention relates to a gas impermeable sealing assembly for a container of pressurised fluid. The invention is particularly applicable to gas impermeable containers such as PET or glass bottles.
1. Technical Problem
In order to achieve a significant shelf life, a container of pressurised fluid must be sealed in such a manner that gas cannot escape. For larger containers of, for example, carbonated drinks, the consumer may not wish to use all of the contents at once. There is therefore a demand for a resealable closure.
2. Prior Art Solutions
A metal crown cork is a conventional gas impermeable sealing assembly. However it is not resealable and is not suitable for a PET bottle.
Typically PET or glass bottles will have an injection-moulded polyethylene cap that screws on to a preformed open neck which is part of the PET or glass bottle. A wad of low density polyethylene sealing material that engages against the open neck is provided in the top of the cap. Alternatively, the cap may have a depending valve, which seats inside the neck.
When PET containers are filled at a pressure of 3 to 5 bar, the CO2 permeability of the container cap assembly of the prior art is not as good as that provided by a glass bottle with a metal cap. The shelf life is also determined by the penetration of oxygen into the container and by migration of aldehydes from the PET container into the product. Glass provides a perfect oxygen barrier. PET, depending upon the treatment, has an oxygen permeability which varies between 0.0049 cm3 per day to 0.0012 cm3 per day. Technical advances continue to be made which reduce the level of oxygen and carbon dioxide permeability of PET containers to levels comparable to that of glass and tin cans. The closure of the container is therefore now of fundamental importance in maintaining the overall performance of the container as an oxygen and carbon dioxide barrier.
It has been proposed to place an oxygen scavenger in the lining of the cap.
Ascorbates such as NaSO2 which oxidises to NaSO3 have been employed for this purpose. These ascorbates are relatively expensive and increase the overall cost of the container, which can be commercially critical.
Aluminium seals have been proposed but once removed there is no method of resealing such a closure and maintaining pressure insider the container.
There is, therefore, a technical problem to provide a cap that acts as an oxygen and carbon dioxide barrier such that the overall container in its ex-factory sealed condition loses less than 10% of carbon dioxide over a six-month shelf life. At present a standard untreated PET bottle loses 36% of the carbon dioxide after six months. This can be reduced by the use of so-called “multi-layered” PET bottles or bottles which have been treated with an amorphous carbon treatment on their internal surface, such as that offered by the French company, Sidel (RTM) under the ACTIS trade mark.